This invention relates to frequency modulation (FM) systems for receiving voice and data signals and more particularly to the detection and suppression of interference in such systems.
A variety of apparatus and techniques currently exist for processing FM signals in communication systems. In those situations in which the signal-of-interest has co-channel interference or more than one co-channel signal is desired, prior art solutions to such instances may not effectively separate the signals, and where separable, generally require duplicative equipment and increased processing time.
A typical receiver is comprised of one or more phase-locked loops (PLL) that may include fixed signal limits. The PLL may take the form of an analog or digital implementation, depending upon design choice and available resources. Generally, a PLL is dedicated to suppress noise by locking to and filtering the signal. The PLL contains a voltage-controlled oscillator, whose output could be multiplied with the input signal and integrated to establish a relative measure of noise level in the input signal. Additional PLLs may be included that are bandwidth tailored for the specific demodulation scheme desired. Some such systems may include a threshold (or thresholds) above which the detected noise level will cause the output to be squelched.
Yet another approach to signal processing is based upon the Hilbert transform. The Hilbert transform is a well-known mathematical principle, commonly used in signal processing, that is here used to translate the IF signal carrier to complex baseband in-phase (I) and quadrature-phase (Q) signal components. The Hilbert transform is often implemented from a real sampling of the IF signal carrier and then separated into I and Q channels, sometimes without quadrature carrier multiplies. Not using quadrature carrier multiplies relies on a fixed sample rate to carrier frequency relationship which can suffer from degraded performance, especially when the input signal is offset in frequency by Doppler shift and reference inaccuracies. The baseband signal demodulator then essentially becomes a hardware implementation of the premise that an FM signal can be processed from the derivative of its carrier phase.
Yet another prior art approach to multi-path interference problems, is based on the use of a plurality of canceler stages. Each canceler stage delays the input signal pulse by a fixed interval, typically equal to the pulse width. Next, the level of the undelayed main pulse is adjusted to equal the level of the first reflection pulse. This value is then subtracted from the delayed pulse. The process repeats for additional stages, as design requirements indicate, until the remaining reflections are suppressed an adequate amount. This approach is only good for well defined multi-path delay times and relatively short duration pulse signals.
Accordingly, a need exists for a communication architecture and processing method that is capable of efficiently handling arbitrary co-channel signal interference with minimal hardware requirements.